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                <ol class="chapter"><li class="chapter-item expanded affix "><a href="翻译说明.html">翻译说明</a></li><li class="chapter-item expanded affix "><a href="introduction.html">介绍</a></li><li class="chapter-item expanded "><a href="notation.html"><strong aria-hidden="true">1.</strong> 表义符</a></li><li class="chapter-item expanded "><a href="lexical-structure.html"><strong aria-hidden="true">2.</strong> 词法结构</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="input-format.html"><strong aria-hidden="true">2.1.</strong> 输入格式</a></li><li class="chapter-item expanded "><a href="keywords.html"><strong aria-hidden="true">2.2.</strong> 关键字</a></li><li class="chapter-item expanded "><a href="identifiers.html"><strong aria-hidden="true">2.3.</strong> 标识符</a></li><li class="chapter-item expanded "><a href="comments.html"><strong aria-hidden="true">2.4.</strong> 注释</a></li><li class="chapter-item expanded "><a href="whitespace.html"><strong aria-hidden="true">2.5.</strong> 空白符</a></li><li class="chapter-item expanded "><a href="tokens.html"><strong aria-hidden="true">2.6.</strong> token</a></li></ol></li><li class="chapter-item expanded "><a href="macros.html"><strong aria-hidden="true">3.</strong> 宏</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="macros-by-example.html"><strong aria-hidden="true">3.1.</strong> 声明宏</a></li><li class="chapter-item expanded "><a href="procedural-macros.html"><strong aria-hidden="true">3.2.</strong> 过程宏</a></li></ol></li><li class="chapter-item expanded "><a href="crates-and-source-files.html"><strong aria-hidden="true">4.</strong> crate 和源文件</a></li><li class="chapter-item expanded "><a href="conditional-compilation.html"><strong aria-hidden="true">5.</strong> 条件编译</a></li><li class="chapter-item expanded "><a href="items.html"><strong aria-hidden="true">6.</strong> 程序项</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="items/modules.html"><strong aria-hidden="true">6.1.</strong> 模块</a></li><li class="chapter-item expanded "><a href="items/extern-crates.html"><strong aria-hidden="true">6.2.</strong> 外部crate</a></li><li class="chapter-item expanded "><a href="items/use-declarations.html"><strong aria-hidden="true">6.3.</strong> use声明</a></li><li class="chapter-item expanded "><a href="items/functions.html"><strong aria-hidden="true">6.4.</strong> 函数</a></li><li class="chapter-item expanded "><a href="items/type-aliases.html"><strong aria-hidden="true">6.5.</strong> 类型别名</a></li><li class="chapter-item expanded "><a href="items/structs.html"><strong aria-hidden="true">6.6.</strong> 结构体</a></li><li class="chapter-item expanded "><a href="items/enumerations.html"><strong aria-hidden="true">6.7.</strong> 枚举</a></li><li class="chapter-item expanded "><a href="items/unions.html"><strong aria-hidden="true">6.8.</strong> 联合体</a></li><li class="chapter-item expanded "><a href="items/constant-items.html"><strong aria-hidden="true">6.9.</strong> 常量项</a></li><li class="chapter-item expanded "><a href="items/static-items.html"><strong aria-hidden="true">6.10.</strong> 静态项</a></li><li class="chapter-item expanded "><a href="items/traits.html"><strong aria-hidden="true">6.11.</strong> trait</a></li><li class="chapter-item expanded "><a href="items/implementations.html"><strong aria-hidden="true">6.12.</strong> 实现</a></li><li class="chapter-item expanded "><a href="items/external-blocks.html"><strong aria-hidden="true">6.13.</strong> 外部块</a></li><li class="chapter-item expanded "><a href="items/generics.html"><strong aria-hidden="true">6.14.</strong> 泛型参数</a></li><li class="chapter-item expanded "><a href="items/associated-items.html"><strong aria-hidden="true">6.15.</strong> 关联程序项</a></li></ol></li><li class="chapter-item expanded "><a href="attributes.html"><strong aria-hidden="true">7.</strong> 属性</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="attributes/testing.html"><strong aria-hidden="true">7.1.</strong> 测试</a></li><li class="chapter-item expanded "><a href="attributes/derive.html"><strong aria-hidden="true">7.2.</strong> 派生</a></li><li class="chapter-item expanded "><a href="attributes/diagnostics.html"><strong aria-hidden="true">7.3.</strong> 诊断</a></li><li class="chapter-item expanded "><a href="attributes/codegen.html"><strong aria-hidden="true">7.4.</strong> 代码生成</a></li><li class="chapter-item expanded "><a href="attributes/limits.html"><strong aria-hidden="true">7.5.</strong> 极限值设置</a></li><li class="chapter-item expanded "><a href="attributes/type_system.html"><strong aria-hidden="true">7.6.</strong> 类型系统</a></li></ol></li><li class="chapter-item expanded "><a href="statements-and-expressions.html"><strong aria-hidden="true">8.</strong> 语句和表达式</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="statements.html"><strong aria-hidden="true">8.1.</strong> 语句</a></li><li class="chapter-item expanded "><a href="expressions.html"><strong aria-hidden="true">8.2.</strong> 表达式</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="expressions/literal-expr.html"><strong aria-hidden="true">8.2.1.</strong> 字面量表达式</a></li><li class="chapter-item expanded "><a href="expressions/path-expr.html"><strong aria-hidden="true">8.2.2.</strong> 路径表达式</a></li><li class="chapter-item expanded "><a href="expressions/block-expr.html"><strong aria-hidden="true">8.2.3.</strong> 块表达式</a></li><li class="chapter-item expanded "><a href="expressions/operator-expr.html"><strong aria-hidden="true">8.2.4.</strong> 运算符表达式</a></li><li class="chapter-item expanded "><a href="expressions/grouped-expr.html"><strong aria-hidden="true">8.2.5.</strong> 分组表达式</a></li><li class="chapter-item expanded "><a href="expressions/array-expr.html"><strong aria-hidden="true">8.2.6.</strong> 数组和索引表达式</a></li><li class="chapter-item expanded "><a href="expressions/tuple-expr.html"><strong aria-hidden="true">8.2.7.</strong> 元组和索引表达式</a></li><li class="chapter-item expanded "><a href="expressions/struct-expr.html"><strong aria-hidden="true">8.2.8.</strong> 结构体表达式</a></li><li class="chapter-item expanded "><a href="expressions/call-expr.html"><strong aria-hidden="true">8.2.9.</strong> 调用表达式</a></li><li class="chapter-item expanded "><a href="expressions/method-call-expr.html"><strong aria-hidden="true">8.2.10.</strong> 方法调用表达式</a></li><li class="chapter-item expanded "><a href="expressions/field-expr.html"><strong aria-hidden="true">8.2.11.</strong> 字段访问表达式</a></li><li class="chapter-item expanded "><a href="expressions/closure-expr.html"><strong aria-hidden="true">8.2.12.</strong> 闭包表达式</a></li><li class="chapter-item expanded "><a href="expressions/loop-expr.html"><strong aria-hidden="true">8.2.13.</strong> 循环表达式</a></li><li class="chapter-item expanded "><a href="expressions/range-expr.html"><strong aria-hidden="true">8.2.14.</strong> 区间表达式</a></li><li class="chapter-item expanded "><a href="expressions/if-expr.html"><strong aria-hidden="true">8.2.15.</strong> if 和 if let 表达式</a></li><li class="chapter-item expanded "><a href="expressions/match-expr.html"><strong aria-hidden="true">8.2.16.</strong> 匹配表达式</a></li><li class="chapter-item expanded "><a href="expressions/return-expr.html"><strong aria-hidden="true">8.2.17.</strong> 返回表达式</a></li><li class="chapter-item expanded "><a href="expressions/await-expr.html"><strong aria-hidden="true">8.2.18.</strong> 等待(await)表达式</a></li></ol></li></ol></li><li class="chapter-item expanded "><a href="patterns.html"><strong aria-hidden="true">9.</strong> 模式</a></li><li class="chapter-item expanded "><a href="type-system.html"><strong aria-hidden="true">10.</strong> 类型系统</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="types.html"><strong aria-hidden="true">10.1.</strong> 类型</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="types/boolean.html"><strong aria-hidden="true">10.1.1.</strong> 布尔型</a></li><li class="chapter-item expanded "><a href="types/numeric.html"><strong aria-hidden="true">10.1.2.</strong> 数字型</a></li><li class="chapter-item expanded "><a href="types/textual.html"><strong aria-hidden="true">10.1.3.</strong> 字符型</a></li><li class="chapter-item expanded "><a href="types/never.html"><strong aria-hidden="true">10.1.4.</strong> never类型</a></li><li class="chapter-item expanded "><a href="types/tuple.html"><strong aria-hidden="true">10.1.5.</strong> 元组</a></li><li class="chapter-item expanded "><a href="types/array.html"><strong aria-hidden="true">10.1.6.</strong> 数组</a></li><li class="chapter-item expanded "><a href="types/slice.html"><strong aria-hidden="true">10.1.7.</strong> 切片</a></li><li class="chapter-item expanded "><a href="types/struct.html"><strong aria-hidden="true">10.1.8.</strong> 结构体</a></li><li class="chapter-item expanded "><a href="types/enum.html"><strong aria-hidden="true">10.1.9.</strong> 枚举</a></li><li class="chapter-item expanded "><a href="types/union.html"><strong aria-hidden="true">10.1.10.</strong> 联合体</a></li><li class="chapter-item expanded "><a href="types/function-item.html"><strong aria-hidden="true">10.1.11.</strong> 函数项类型</a></li><li class="chapter-item expanded "><a href="types/closure.html"><strong aria-hidden="true">10.1.12.</strong> 闭包</a></li><li class="chapter-item expanded "><a href="types/pointer.html"><strong aria-hidden="true">10.1.13.</strong> 指针型</a></li><li class="chapter-item expanded "><a href="types/function-pointer.html"><strong aria-hidden="true">10.1.14.</strong> 函数指针</a></li><li class="chapter-item expanded "><a href="types/trait-object.html"><strong aria-hidden="true">10.1.15.</strong> trait对象</a></li><li class="chapter-item expanded "><a href="types/impl-trait.html"><strong aria-hidden="true">10.1.16.</strong> 实现trait</a></li><li class="chapter-item expanded "><a href="types/parameters.html"><strong aria-hidden="true">10.1.17.</strong> 类型参数</a></li><li class="chapter-item expanded "><a href="types/inferred.html"><strong aria-hidden="true">10.1.18.</strong> 推断型</a></li></ol></li><li class="chapter-item expanded "><a href="dynamically-sized-types.html"><strong aria-hidden="true">10.2.</strong> 动态尺寸类型(DST)</a></li><li class="chapter-item expanded "><a href="type-layout.html"><strong aria-hidden="true">10.3.</strong> 类型布局 </a></li><li class="chapter-item expanded "><a href="interior-mutability.html"><strong aria-hidden="true">10.4.</strong> 内部可变性</a></li><li class="chapter-item expanded "><a href="subtyping.html"><strong aria-hidden="true">10.5.</strong> 子类型和型变</a></li><li class="chapter-item expanded "><a href="trait-bounds.html"><strong aria-hidden="true">10.6.</strong> trait约束及其生存期约束</a></li><li class="chapter-item expanded "><a href="type-coercions.html"><strong aria-hidden="true">10.7.</strong> 类型自动强转</a></li><li class="chapter-item expanded 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非安全函数</a></li><li class="chapter-item expanded "><a href="unsafe-blocks.html"><strong aria-hidden="true">15.2.</strong> 非安全代码块</a></li><li class="chapter-item expanded "><a href="behavior-considered-undefined.html"><strong aria-hidden="true">15.3.</strong> 未定义行为</a></li><li class="chapter-item expanded "><a href="behavior-not-considered-unsafe.html"><strong aria-hidden="true">15.4.</strong> 不被认为是非安全的行为</a></li></ol></li><li class="chapter-item expanded "><a href="const_eval.html" class="active"><strong aria-hidden="true">16.</strong> 常量求值</a></li><li class="chapter-item expanded "><a href="abi.html"><strong aria-hidden="true">17.</strong> ABI</a></li><li class="chapter-item expanded "><a href="runtime.html"><strong aria-hidden="true">18.</strong> Rust运行时</a></li><li class="chapter-item expanded "><a href="appendices.html"><strong aria-hidden="true">19.</strong> 附录</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="macro-ambiguity.html"><strong aria-hidden="true">19.1.</strong> 宏定义规范</a></li><li class="chapter-item expanded "><a href="influences.html"><strong aria-hidden="true">19.2.</strong> 影响来源</a></li><li class="chapter-item expanded "><a href="glossary.html"><strong aria-hidden="true">19.3.</strong> 术语表</a></li><li class="chapter-item expanded "><a href="本书术语翻译对照表.html"><strong aria-hidden="true">19.4.</strong> 本书术语翻译对照表</a></li></ol></li></ol>
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                        <h1><a class="header" href="#constant-evaluation" id="constant-evaluation">Constant evaluation</a></h1>
<h1><a class="header" href="#常量求值" id="常量求值">常量求值</a></h1>
<blockquote>
<p><a href="https://github.com/rust-lang/reference/blob/master/src/const_eval.md">const_eval.md</a><br />
commit:  8425f5bad3ac40e807e3f75f13b989944da28b62 <br />
本章译文最后维护日期：2021-4-5</p>
</blockquote>
<p>常量求值是在编译过程中计算[表达式][<a href="expressions.html">expressions</a>]结果的过程。（不是所有表达式都可以在编译时求值，也就是说）只有全部表达式的某个子集可以在编译时求值。</p>
<h2><a class="header" href="#constant-expressions" id="constant-expressions">Constant expressions</a></h2>
<h2><a class="header" href="#常量表达式" id="常量表达式">常量表达式</a></h2>
<p>某些形式的表达式（被称为常量表达式）可以在编译时求值。在<a href="#const-context">常量(const)上下文</a>中，常量表达式是唯一允许的表达式，并且总是在编译时求值。在其他地方，比如 <a href="statements.html#let-statements">let语句</a>，常量表达式<em>可以</em>在编译时求值，但不能保证总能在此时求值。如果值必须在编译时求得（例如在常量上下文中），则像<a href="expressions/array-expr.html#array-and-slice-indexing-expressions">数组索引</a>越界或<a href="expressions/operator-expr.html#overflow">溢出</a>这样的行为都是编译错误。如果不是必须在编译时求值，则这些行为在编译时只是警告，但它们在运行时可能会触发 panic。</p>
<p>下列表达式中，只要它们的所有操作数都是常量表达式，并且求值/计算不会引起任何 <a href="destructors.html"><code>Drop::drop</code></a>函数的运行，那这些表达式就是常量表达式。</p>
<ul>
<li><a href="expressions/literal-expr.html">字面量</a>。</li>
<li><a href="items/generics.html">常量参数</a>。</li>
<li>指向<a href="items/functions.html">函数项</a>和<a href="items/constant-items.html">常量项</a>的<a href="expressions/path-expr.html">路径</a>。不允许递归地定义常量项。</li>
<li>指向<a href="items/static-items.html">静态项</a>的路径。这种路径只允许出现在静态项的初始化器中。</li>
<li><a href="expressions/tuple-expr.html">元组表达式</a>。</li>
<li><a href="expressions/array-expr.html">数组表达式</a>。</li>
<li><a href="expressions/struct-expr.html">结构体</a>表达式。</li>
<li><a href="expressions/block-expr.html">块表达式</a>，包括非安全(<code>unsafe</code>)块。
<ul>
<li><a href="statements.html#let-statements">let语句</a>以及类似这样的不可反驳型<a href="patterns.html">模式</a>绑定，包括可变绑定。</li>
<li><a href="expressions/operator-expr.html#assignment-expressions">赋值表达式</a></li>
<li><a href="expressions/operator-expr.html#compound-assignment-expressions">复合赋值表达式</a></li>
<li><a href="statements.html#expression-statements">表达式语句</a></li>
</ul>
</li>
<li><a href="expressions/field-expr.html">字段</a>表达式。</li>
<li>索引表达式，长度为 <code>usize</code> 的<a href="expressions/array-expr.html#array-and-slice-indexing-expressions">数组索引</a>或<a href="types/slice.html">切片</a>。</li>
<li><a href="expressions/range-expr.html">区间表达式</a>。</li>
<li>未从环境捕获变量的<a href="expressions/closure-expr.html">闭包</a>。</li>
<li>在整型、浮点型、布尔型(<code>bool</code>)和字符型(<code>char</code>)上做的各种内置运算，包括：<a href="expressions/operator-expr.html#negation-operators">取反</a>、<a href="expressions/operator-expr.html#arithmetic-and-logical-binary-operators">算术</a>、<a href="expressions/operator-expr.html#arithmetic-and-logical-binary-operators">逻辑</a>、<a href="expressions/operator-expr.html#comparison-operators">比较</a> 或 <a href="expressions/operator-expr.html#lazy-boolean-operators">惰性布尔</a>运算。</li>
<li>排除借用类型为<a href="interior-mutability.html">内部可变借用</a>的共享<a href="expressions/operator-expr.html#borrow-operators">借用</a>表达式。</li>
<li>排除解引用裸指针的<a href="expressions/operator-expr.html#the-dereference-operator">解引用操作</a>。8425f5bad3ac40e807e3f75f13b989944da28b62
<ul>
<li>指针到地址的强制转换，</li>
<li>函数指针到地址的强制转换，和</li>
<li>到 trait对象的非固定尺寸类型强换(unsizing casts)。</li>
</ul>
</li>
<li>调用<a href="items/functions.html#const-functions">常量函数</a>和常量方法。</li>
<li><a href="expressions/loop-expr.html#infinite-loops">loop</a>, <a href="expressions/loop-expr.html#predicate-loops">while</a> 和 <a href="expressions/loop-expr.html#predicate-pattern-loops"><code>while let</code></a> 表达式。</li>
<li><a href="expressions/if-expr.html#if-expressions">if</a>, <a href="expressions/if-expr.html#if-let-expressions"><code>if let</code></a> 和 [匹配(match)] 表达式。</li>
</ul>
<h2><a class="header" href="#const-context" id="const-context">Const context</a></h2>
<h2><a class="header" href="#常量上下文" id="常量上下文">常量上下文</a></h2>
<p>下述位置是<em>常量上下文</em>：</p>
<ul>
<li><a href="types/array.html">数组类型的长度表达式</a></li>
<li><a href="expressions/array-expr.html">分号分隔的数组创建形式中的长度表达式</a></li>
<li>下述表达式的初始化器(initializer)：
<ul>
<li><a href="items/constant-items.html">常量项</a></li>
<li><a href="items/static-items.html">静态项</a></li>
<li><a href="items/enumerations.html#custom-discriminant-values-for-fieldless-enumerations">枚举判别值</a></li>
</ul>
</li>
<li><a href="items/generics.html#const-generics">常量型泛型实参</a></li>
</ul>
<h2><a class="header" href="#const-functions" id="const-functions">Const Functions</a></h2>
<h2><a class="header" href="#常量函数" id="常量函数">常量函数</a></h2>
<p>*常量函数(const fn)*可以在常量上下文中调用。给一个函数加一个常量(<code>const</code>)标志对该函数的任何现有的使用都没有影响，它只限制参数和返回可以使用的类型，并防止在这两个位置上使用不被允许的表达式类型。程序员可以自由地用常量函数去做任何用常规函数能做的事情。</p>
<p>当从常量上下文中调用这类函数时，编译器会在编译时解释该函数。这种解释发生在编译目标环境中，而不是在当前主机环境中。因此，如果是针对一个 <code>32</code> 位目标系统进行编译，那么 <code>usize</code> 就是 <code>32</code> 位，这与在一个 <code>64</code> 位还是在一个 <code>32</code> 位主机环境中进行编译动作无关。</p>
<p>常量函数有各种限制以确保其可以在编译时可被求值。因此，例如，不可以将随机数生成器编写为常量函数。在编译时调用常量函数将始终产生与运行时调用它相同的结果，即使多次调用也是如此。这个规则有一个例外：如果在极端情况下执行复杂的浮点运算，那么可能得到（非常轻微）不同的结果。建议不要让数组长度和枚举判别值/式依赖于浮点计算。</p>
<p>常量上下文有，但常量函数不具备的显著特性有：</p>
<ul>
<li>浮点运算
<ul>
<li>（在常量函数中，）处理浮点值就像处理只有 <code>Copy</code> trait约束的泛型参数一样，不能用它们做任何事，只能复制/移动它们。</li>
</ul>
</li>
<li>trait对象(<code>dyn Trait</code>)/动态分发类型</li>
<li>泛型参数上除 <code>Sized</code> 之外的泛型约束</li>
<li>比较裸指针</li>
<li>访问联合体字段</li>
<li>调用 <a href="https://doc.rust-lang.org/std/mem/fn.transmute.html"><code>transmute</code></a>。</li>
</ul>
<p>相反，以下情况在常量函数中是有可能的，但在常量上下文中则不可能：</p>
<ul>
<li>使用泛型类型和生存期参数。
<ul>
<li>常量上下文允许有限地使用<a href="items/generics.html#const-generics">常量型泛型形参</a>。</li>
</ul>
</li>
</ul>

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